Wide band spiral antenna with reflective cavities of varied sizes



1967 G. DUBOST ET AL 3,358,288

WIDE BAND SPIRAL ANTENNA WITH REFLECTIVE CAVITIES D SIZES Filed 1964 OF VARIE June 26,

United States Patent 3,358,288 WIDE BAND SPIRAL ANTENNA WITH REFLEC- TIVE CAVI'IIES 0F VARIED SIZES Grard Dubost and Pierre Amiot, Paris, France, assignors to CSF-Compagnie Generale de Telegraphic Sans Fil, a corporation of France Filed June 26, 1964, Ser. No. 378,244 Claims priority, application France, July 4, 1963, 940,361 1 Claim. (Cl. 343895) Double spiral antennas are known in high frequency techniques. They form a double branch radiating system. The two branches lie in the same plane and are formed as spirals of Archimedes, which are symmetrical with respect to a point. They are fed in phase opposition.

Such antennas are bidirectional, i.e. have two directions of maximum radiation, which directions are symmetrical with respect to the antenna plane.

As is known, one of the radiation directions can be eliminated by placing, behind the double spiral, a cylindrical cavity, the axis of which is at right angles with the antenna plane, the dimension of this cavity normally to the antenna plane being equal to a quarter Wavelength at the average operating frequency.

Such antennas have a passing band of the order of one octave.

It is an object of the invention, to provide an antenna of the above type, whose passing band is slightly better than two octaves.

An antenna according to the invention comprises, behind its plane, a cavity of revolution about an axis perpendicular to the antenna plane, the cavity being built up by a plurality of cylindrical rings of difference depths.

According to a preferred embodiment of the invention the ratio of the average radius of each cylindrical ring to its depth is 2/1r.

The invention will be better understood from the following description and accompanying drawing in which:

FIG. 1 is a perspective View of the double spiral antenna embodying the invention;

FIG. 2 is a sectional view of a first embodiment of the cavity embodied in an antenna according to the invention;

FIG. 3 is a plane view of the cavity shown in FIG. 2; and

FIG. 4 is a sectional view of a further embodiment of the cavity.

The antenna shown in FIG. 1 is formed as a printed circuit on a Teflon sheet 1.

It comprises two spirals 2 and 3, which are fed in phase opposition by a bifilar line 4.

As is known, such antennas are capable of radiating energy within a wide frequency band. Each radiation frequency is radiated by an antenna portion closely approximating a circle, whose average diameter is A/1r.

3,358,288 Patented Dec. 12, 1967 Thus the antenna may be viewed as formed by a set of rings of an average diameter ranging between x /w and x /w, k and A being respectively the minimum and the maximum wavelengths.

To avoid any radiation on one side of the antenna, it is known to place at that side a cylindrical cavity.

This cavity reflects the radiation in the opposite direction with the same phase as the radiation existing in this latter direction, if the cavity depth equals a quarter of the operating wavelength. Of course the use of this cavity limits the passing band of the antenna. According to the invention, a particular cavity is used. This cavity is shown in sectional view in FIG. 2 and in FIG. 3 in plane view. Ths cavity is fixed to theTefion plate 1. It comprises, in the embodiment shown, four cylindrical rings, 5, 6, 7 and 8, whose respective depths are equal to their average radius time 1r/2.

In other words, the portions of the two spirals, respectively facing cylindrical rings 5, 6, 7 and 8 radiating a narrow band of frequencies, centered on a frequency fc corresponding to the wavelength h will be facing a ring whose depth is p= /4.

FIG. 4 shows a modification of the cavity of FIG. 2.

The cavity shown in FIG. 4 comprises a central part 10, of depth M4, for the higher operating frequencies and an annular cavity 20 of suitable diameter and depth to reflect the radiation at lower frequencies. Conic walls 30 and 40 attenuate parasitic resonances.

Such a cavity has been designed for a band of frequency ranging from 2.5 to 11 gc./s.

What is claimed is:

An antenna for radiating a very broad frequency band, comprising in combination: two branches extending in the same plane, shaped as spirals of Archimedes and symmetrical with respect to a point; a cavity resonator behind said plane, shaped as a body of revolution about an axis perpendicular to said plane, said cavity having a central portion of a first predetermined depth for reflecting the higher frequencies, and an annular portion, having a second predetermined depth for reflecting the lower frequencies of the passing band of said antenna; said first and said second average depths being substantially proportional to the lower and the higher wavelengths to be radiated and said central and annular portions having conic walls.

References Cited UNITED STATES PATENTS 4/1964 Wheeler 343-895 6/1965 Cox et a1 343895 

